Our brain was once thought to be “immune privileged” in that a wall, called the blood-brain barrier, isolated the central nervous system from all the hubbub that goes on in the immune system. But research proved that notion to be false a while ago. A case in point is a recent study, which found that healthy people’s ability to remember spatial information was affected when their immune system was given a jolt.

Inflammation is typically seen in Alzheimer’s disease when damaged cells and harmful protein deposits trigger an immune response in the brain that only makes cognitive matters even worse. Even healthy elderly people who fall sick from a bug can suddenly spiral into a confused, disoriented state known as delirium, which rarely ends with full mental recovery. In addition to neurological disorders and age-related vulnerability, inflammation has been found to wreak havoc on memory systems of healthy rodents and impairs their ability to remember things.

The current study, published in Biological Psychiatry, was carried out by researchers at UK’s Brighton and Sussex Medical School, and is a first in using positron emission tomography (PET) and virtual memory tests to show that younger, healthy people are also vulnerable to memory glitches when they become sick. "This study suggests that catching a cold or the flu, which leads to inflammation in the brain, could impair our memory," lead author of the study Dr. Neil Harrison said in a press release.

Harrison and his team scanned 20 people before and after they were injected with either harmless salt water or a typhoid vaccination that induced inflammation. Subsequent PET scans detected a reduction in glucose (sugar) metabolism in the brain’s memory hub, called the medial temporal lobe, only in those who got jabbed with the vaccination. These lesser fortunate participants also performed worse on a virtual spatial memory test that involved a maze but did fine on a tricky mirror-tracing task. The authors attributed this selective memory effect on the fact that glucose metabolism was lowered in brain areas that distinctly mediate spatial memory. Altogether, the authors showed that our memory center is quite sensitive to even mild challenges to our immune system. More importantly, the findings should be a red flag for certain vulnerable, elderly individuals since infections like influenza can open the Pandora’s box of mental decline into dementia.

This work validates a well-known study carried out last year in Manhattan, which found that lower scores on tests of mental acuity were associated with the degree of exposure to five common viruses and bacteria: cytomegalovirus, herpes 1 and 2, Helicobacter pylori, and Chlamydia pneumoniae. This trend was strongest among those who didn’t have a high school diploma, who were physically inactive, and women. The lead author of the study, Dr. Mira Katan of Columbia University, couldn’t explain the exact reason for this brain-immune association, but proposed that the pathogens were likely toxic to nerves.

Source: Harrison N A, Doeller C F, Voon V et al. Peripheral Inflammation Acutely Impairs Human Spatial Memory via Actions on Medial Temporal Lobe Glucose Metabolism. Biological Psychiatry. 2014.